The use of temperature-responsive paint compositions for diverse applications, such as in the electrical art, has been disclosed, for example in U.S. Pat. No. 1,924,793 to Laske wherein there is formulated a paint comprised of a mixture of litharge with thiourea. The temperature-responsive characteristics of mesomorphic compositions, and in paticular, the cholesteric and nematic types, have been recognized in the prior art, such as disclosed in U.S. Pat. No. 3,576,761 to Davis. The encapsulation of mesomorphic compositions is disclosed, inter alia, in U.S. Pat. Nos. 2,800,457 and 3,341,466. Incorporation of mesomorphic compositions in polymer matrixes is disclosed in U.S. Pat. Nos. 3,600,060 and 3,620,889.
In U.S. Pat. No. 3,697,297 to Churchill et al, there is disclosed a process for the microencapsulation of mesomorphic compositions in a thermoplastic polymer matrix to form minute capsules which when subjected to conductive heat transfer exhibits a reversible irridescence color condition indicative of a certain characteristic temperature range. Such temperature range varies in accordance with the types and mixtures of the mesomorphic compositions.
The use of mesomorphic compositions generally recognizes the physical property of reversibility when transitting the temperature range between the truly crystalline state and the isotropic state, although the hereinabove mentioned Davis reference suggests the use of mesomorphic compositions and in particular the cholesteric types, to record a permanent temperature change by the use of oil soluble dyes for low temperature application, e.g. below about 100.degree. C.
It has been proposed to use materials dissolved in an organic solvent, however once applied to a substrate with concomitant evaporation of the solvent results in a coating which may be readily removed by brushing thereby obviating any consideration for permanent uses.
Mesomorphic or "liquid crystalline" materials have long been known and have been classified into three types: smectic, nematic, and cholesteric. The cholesteric types exhibit a number of optical properties, one of which being the scattering of white light, varying with temperature over a certain range, depending upon the particular cholesteric material. The formulation of mesomorphic compositions to achieve a desired temperature at which the mesomorphic composition clears is disclosed in the hereinabove mentioned U.S. Pat. No. 3,620,899 to Baltzer.
In a copending application Ser. No. 363,909 there is disclosed a process for forming micro-capsules of a liquid crystalline composition having a preselected isotropic temperature whereby once the liquid crystalline composition transits into the isotropic phase, the liquid crystalline composition thereafter essentially visually indicates such a transition notwithstanding a subsequent lowering of temperature to below the isotropic temperature. The micro-capsules of the liquid crystalline composition are sized from 5 to 20 microns, and preferably 5 to 10 microns, and are comprised of a water-soluble encapsulating material surrounding the liquid crystalline composition wherein the encapsulating material is formed about the liquid crystalline composition at a temperature above its isotropic temperature. In one embodiment thereof the micro-capsules are case-hardened by contact with a cross-linking or dehydrating agent following microencapsulation.
Liquid crystalline compositions are relatively expensive and are relatively sensitive to contamination and may lose their temperature-responsive property. Micro-encapsulations techniques requiring time controlled processing operations, as well as careful attention to processing conditions to produce effective micro-capsules of liquid crystalline compositions. The liquid crystalline compositions are temperature-responsive to narrow and generally lower temperature ranges whereas many industrial applications only require a response to broader temperature ranges and at higher temperatures.